The remaining parties to the 2015 Iran nuclear deal -- Britain, China, France, Germany and Russia as well as Iran, held talks for about an hour at a luxury hotel in the Austrian capital.

The Russian ambassador to the UN in Vienna, Mikhail Ulyanov, tweeted after Friday's session that "the participants agreed on the need to intensify the process".

"The delegations seem to be ready to stay in Vienna as long as necessary to achieve the goal," Ulyanov said.

One diplomatic source told AFP before Friday's talks that while "nothing was guaranteed" at the negotiations, they are "on the right track".

The source expressed hope that the latest talks could be the "final round".

The aim is a return to full compliance with the 2015 accord, known as the JCPOA, which has been slowly disintegrating since former US president Donald Trump withdrew from it in 2018 and Tehran responded by breaking the deal's limits on its nuclear activities.

Trump's successor Joe Biden has said he wishes to rejoin the JCPOA, and an American official said on Thursday an agreement was "possible" in the coming weeks, before Iran's June 18 election.

The US is taking part in the talks indirectly, with European diplomats acting as intermediaries between the Americans and Iranians.

The talks are seeking to find a way to lift sanctions on Iran reimposed by Trump, and for Iran to come back into line with the JCPOA's limits on its nuclear activities.

Besides the Iran election, another source of time pressure is the looming expiration of a three-month agreement between Iran and the UN nuclear watchdog, the International Atomic Energy Agency (IAEA).

The agreement announced in February allows the IAEA to maintain a level of oversight of Iran's nuclear facilities despite Tehran's suspension of some inspections by the body.

Under the agreement, Iran pledged to keep recordings "of some activities and monitoring equipment" and hand them over to the IAEA as and when US sanctions are lifted.

IAEA Director General Rafael Grossi on Thursday met the head of the Iranian delegation to the Vienna talks, Deputy Foreign Minister Abbas Araghchi.

In an interview last week with Bloomberg TV, Grossi expressed concern that Iran would destroy the camera data if an agreement was not reached in time, adding: "Let's hope it hasn't happened."

He said that if the temporary agreement expired and the broader diplomatic talks remained stuck, he would return to Tehran to try to broker another compromise.